1. Field of the Invention
The present invention relates to a nonaqueous secondary battery comprising a negative electrode capable of intercalating/deintercalating lithium ion, a positive electrode made of a lithium-containing metal oxide as an active positive electrode material, a nonaqueous electrolyte and a separator for separating the positive electrode and the negative electrode from each other.
2. Description of the Related Art
Recently, electronic apparatus have shown a remarkable reduction of size and weight. Under these circumstances, it has been keenly desired to reduce the size and weight of battery as power supply. Accordingly, a secondary lithium battery such as lithium ion battery has been put to practical use as rechargeable battery having a small weight and high capacity. Such a secondary lithium battery has been used for portable electronic and communications apparatus such as small-sized video camera, portable telephone and note type personal computer.
This type of a secondary lithium battery comprises as an active negative electrode material a carbon-based material capable of intercalating/deintercalating lithium ion, as an active positive electrode material a lithium-containing metal oxide such as LiCoO2, LiNiO2, LiMn2O4 and LiFeO2, and an electrolytic solution obtained by dissolving a lithium salt as a solute in an organic solvent. These components are assembled into a battery. When this battery is charged for the first time, lithium ions eluted from the active positive electrode material enter in the carbon particles to make the battery rechargeable.
Such a nonaqueous secondary battery comprises a nonaqueous electrolyte. Accordingly, as the separator for separating the positive electrode and the negative electrode from each other there has heretofore been used a microporous membrane of a polyolefin-based resin which has a low reactivity with an organic solvent and is inexpensive, such as polyethylene (PE) and polypropylene (PP).
However, a microporous membrane of polyethylene or polypropylene is disadvantageous in that it merely keeps a retainability of electrolyte in its pores and thus cannot fairly retain an electrolytic solution, causing an increase of internal resistivity resulting in a drastic drop of the battery capacity after high temperature storage.
In order to improve the liquid retaining properties of the battery, it has been practiced to use a non-woven cloth of polypropylene (PP) or polyethylene terephthalate (PET) as a separator. However, a non-woven cloth of polypropylene (PP) or polyethylene terephthalate (PET) is disadvantageous in that it causes a drastic drop of the battery capacity after high temperature storage as in the case of microporous membrane.
As a countermeasure, the use of a polyvinylidene fluoride (PVdF) membrane having good liquid retaining properties has been proposed. Since the polyvinylidene fluoride resin membrane can fairly keep retainability of electrolyte and come in close contact with the electrodes, the resulting battery exhibits a lowered internal resistivity and hence improved properties.
Such a nonaqueous secondary battery is normally produced by laminating a positive electrode and a negative electrode with a separator provided interposed therebetween, spirally winding the laminate to prepare an electrode body, and then inserting the electrode body in a cylindrical battery case. However, the polyvinylidene fluoride membrane is disadvantageous in that it has a low mechanical strength and thus can break to cause internal shortcircuiting when spirally wound.
Under these circumstances, the present invention has been worked out to solve the foregoing problems. An object of the invention is to provide a nonaqueous secondary battery which comprises an electrolytic solution having an improved maintenance and undergoes little or no internal shortcircuiting to exhibit excellent high temperature storage properties and overcharging properties.
In order to accomplish the foregoing object of the invention, the nonaqueous secondary battery of the invention comprises a polyvinylidene fluoride resin layer formed on either or both of the surface of the positive electrode and the negative electrode. This polyvinylidene fluoride resin layer exhibits excellent liquid retaining properties and thus can be formed on the surface of the electrodes to reduce the capacity drop during high temperature storage and hence improve the high temperature storage properties of the battery.
As the polyvinylidene fluoride resin there is preferably used a vinylidene fluoride homopolymer or a vinylidene fluoride copolymer of vinylidene with one or more selected from the group consisting of ethylene trifluorochloride, ethylene tetrafluoride, propylene hexafluoride and ethylene.
While the formation of such a polyvinylidene fluoride resin layer on the surface of the electrodes makes it possible to improve the liquid retaining properties of the electrodes, the increase of the thickness of the polyvinylidene fluoride resin layer causes an increase of the internal resistivity of the battery. Therefore, it is necessary that the thickness of the polyvinylidene fluoride resin layer be restricted. It was experimentally confirmed that when the thickness of the polyvinylidene fluoride resin exceeds 10 xcexcm, the resulting battery shows an increase of internal resistivity. Thus, the thickness of the polyvinylidene fluoride resin layer is preferably predetermined to be 10 xcexcm or less. Further, when the thickness of the polyvinylidene fluoride resin layer is too small, the resulting liquid retaining properties are not improved. Thus, the polyvinylidene fluoride resin layer needs to have a thickness great enough to improve the liquid retaining properties of the electrodes. Thus, the lower limit of the thickness of the polyvinylidene fluoride resin layer is preferably predetermined to 2 xcexcm or more.
When a nonaqueous secondary battery comprising a polyvinylidene fluoride resin layer formed on the surface of the electrodes is overcharged, heat runaway occurs if it comprises a separator made of a microporous membrane. However, if such a nonaqueous secondary battery comprises a separator made of a fibrous base material, no heat runaway occurs. This demonstrates that the separator for nonaqueous secondary battery comprising a polyvinylidene fluoride resin layer formed on the surface of the electrodes is preferably formed by a fibrous base material and as such a fibrous base material there is preferably used a non-woven cloth.
The method of fabricating a nonaqueous secondary battery of the invention comprises a steps of: forming a positive electrode and negative electrode including electrode active materials; coating a polyvinylidene fluoride resin layer with a solution at least on one surface of the positive electrode and negative electrode, removing the solution in a polyvinylidene fluoride resin layer; and laminating the positive and negative electrodes so that electrolyte is interposed therebetween.
According to the above structure, surfaces of the electrodes can be contacted with the electrolyte closely. Therefore battery which has an excellent liquid retaining properties and have a high battery capacity after high temperature storage, can be obtained.